Historically our concepts of recombination have been derived from gentical studies of higher organisms. Unfortunately little direct biochemical information from eukaryotes about enzymes and proteins involved in recombination is known. The proposed research has as its goal the elucidation of the mechanism of genetic recombination through biochemical characterization of enzymes and proteins involved in the macromolecular DNA metabolism of the simple eukaryote Ustilago maydis, a yeast like fungus whose genetic system is well characterized. The focus of the proposed study is on the elucidation of the mechanism of action of several enzymes likely to be involved in genetic recombination. These enzymes include a protein that promotes pairing of homologous DNA molecules on reaction that requires ATP, two different topoisomerases and an associated low molecular weight protein, and a recently detected ATP-dependent endonuclease. Mutants known to be defective in various steps in recombination such as gene conversion and crossing over and in the overlapping areas of DNA repair and replication are available to help establish the biological role of the enzymes of interest. The kind of information we hope to provide through research set forth is this proposal deals with mechanisms at the molecular level through which genes are rearranged. We are dealing with molecular reactions whose consequences bear on oncogenic processes in animal cells, certain genetic abnormalities in man and the generation of antibody diversity. Our hope is that the investigations we propose will lead to a greater understanding of the molecular basis of recombination and will illuminate a number of important cellular processes.